(1) Field of the Invention
The present invention relates to a process in which a substance present in a liquid reaction system is reacted with a gaseous substance introduced from the outside in the presence of a catalyst. More particularly, the invention relates to a process in which catalytic reaction of a gaseous substance hardly soluble in a liquid is carried out in a liquid phase-solid phase-gas phase heterogeneous reaction system formed on the surface of a solid catalyst.
(2) Description of the Prior Art
The following techniques can be mentioned as the known technique using a water-repellent catalyst or adsorbent.
(1) Japanese Patent Application Laid-Open Specification No. 155492/75 entitled CATALYST FOR HEAVY WATER-HYDROGEN EXCHANGE discloses a hydrophobic catalyst comprising C-Pt-hydrophobic polymer.
(2) Japanese Patent Publication No. 32800/76 entitled PROCESS FOR EXCHANGE OF HYDROGEN ISOTOPES discloses a process for exchange of hydrogen isotopes in which a catalyst comprising at least one metal of the group VIII of the Periodic Table, which is sealed and coated with a gas-permeable or H.sub.2 O-impermeable material, is used.
(3) Japanese Patent Publication No. 41195/76 entitled PROCESS FOR EXCHANGE OF HYDROGEN ISOTOPES BETWEEN HYDROGEN GAS AND LIQUID WATER discloses a process in which hydrogen isotopes are concentrated by using a catalyst comprising an element of the group VIII of the Periodic Table supported on the surface of a polytetrafluoroethylene carrier.
(4) Japanese Patent Application Laid-Open Specification No. 156297/77 entitled PROCESS FOR EXCHANGE OF HYDROGEN ISOTOPES discloses a double-temperature exchange process in which hydrogen isotopes are exchanged between water and H.sub.2 by contacting them in a counter-current manner with a catalyst having a film of a hydrophobic polymer.
(5) Japanese Patent Application Laid-Open Specification No. 4197/78 entield PROCESS FOR EXCHANGE OF HYDROGEN ISOTOPES discloses a process in which reaction is carried out in a mixed bed comprising a hydrophobic catalyst including a supported noble metal and a hydrophilic carrier,
(6) U.S. Pat. No. 4,061,724 entitled CRYSTALLINE SILICA discloses a process in which organic materials are selectively adsorbed from water by using silica which has been rendered hydrophobic.
(7) Canadian Pat. No. 958,821 REDUCTION OXIDATION PROCESS AND APPARATUS.
(8) Canadian Pat. No. 959,628 REDUCTION OF POLYSULFIDE.
Both Canadian patents disclose oxidation and reduction process using a hydrophobic catalyst whose carrier or substrate is electronically conductive.
As the process for oxidizing ions or molecules in aqueous solutions, there are known: (a) a process in which a water-soluble strong oxidant, such as hydrogen peroxide or a permanganate, is used; (b) a process in which ozone gas is used; (c) a process in which an aqueous solution is contacted with an oxygen-containing gas in the presence of a catalyst (homogeneous metal ion catalyst); and (d) a process in which oxidation is biochemically performed by an aerobic microorganism, and various methods of these known techniques are practically carried out. However, these known techniques involve varous disadvantages. For example, oxidants used in the processes (a) and (b) are ordinarily expensive, and in the process (c), since a homogeneous system is employed, separation of the catalyst from the reaction product or starting reactant is very difficult.
When an oxidation reaction is carried out in an aqueous solution by using an oxygen-containing gas, since the amount of oxygen dissolved in water is very small, the oxidation speed is ordinarily very low. In a gas phase oxidation reaction using oxygen gas, for example, oxidation of hydrocarbons, it is known that the reaction is promoted and easily advanced by a solid catalyst, for example, platinum, manganese oxide or nickel oxide supported on alumina. Ordinary solid catalysts, however, can hardly be used in aqueous solutions or the solid catalysts lose the catalytic activity in aqueous solutions. It is apparent that the reason is that these ordinary solid catalysts are hydrophilic and their surfaces are wetted and covered with water in aqueous solutions. Therefore, oxygen gas acting as the oxidant is not allowed to reach the catalyst surface or the active points on the surface of the catalyst are covered with water molecules, with the result that the catalytic activity is not exerted.
As the process for reducing ions or molecules in an aqueous solution, there are known: (1) a process in which a water-soluble reducing agent, such as sodium sulfite, sodium sulfide or acidic stannous chloride, is used; (2) a process in which a powder of a metal producing hydrogen of the nascent state under acidic or alkaline conditions, such as zinc, aluminum or tin, is used; (3) a process in which a water-soluble reducing gas, such as hydrogen sulfide or sulfurous acid gas, is used; and (4) a process in which a reducing gas hardly soluble in water, such as hydrogen or carbon monoxide, is used. Methods of these techniques are practically carried out but these techniques involve defects. For example, in the processes (1), (2) and (3), undesirable by-products are formed in the reaction mixture liquids, reaction vessels are readily corroded, and the reducing agents used are expensive. Furthermore, in the process (4), since the reducing gas used is hardly soluble in water, high temperatures and high pressures are necessary for completion of the reaction. As means for eliminating this defect by increasing the reaction speed, there has been proposed a process in which a homogeneous catalyst system is used. According to this process, however, no substantial improvement is attained and separation of the catalyst from the reactants is very difficult. Furthermore, even if heterogeneous catalysts are used, since conventional solid catalysts are hydrophilic, the catalyst surface is covered with water and the reducing gas is hardly adsorbed on the catalyst surface, with the result that no substantial catalytic effect can be attained.
Various processes for promoting certain reactions by using water-repellent catalysts are known. A process for concentrating heavy water by the following isotope exchange reaction between water and hydrogen has recently been proposed: EQU H.sub.2 O(l)+HD(g).revreaction.HDO(l)+H.sub.2 (g)
It was reported that a platinum type catalyst which has been rendered water-repellent is effective for this reaction. As means for rendering such catalyst water-repellent, there have been proposed a process in which a hydrophilic Pt-Al.sub.2 O.sub.3 or Pt-active carbon catalyst is coated with a silicone oil (see Japanese Patent Publication No. 32800/76) or with a polytetrafluoroethylene resin (see Japanese Patent Publication No. 41195/76 and Japanese Patent Application Laid-Open Specification No. 155492/75), and a process in which Pt is stuck to a water-repellent organic polymer (see Japanese Patent Publication No. 41195/76). As another process using a water-repellent catalyst, there are known a process in which oxygen and hydrogen formed while a secondary battery is used are reacted in the gas phase and converted to water (the catalyst is rendered water-repellent to prevent the catalyst from being wetted with splashes of an electrolyte) and a process in which hydrogen iodide (HI) is decomposed (it is reported that a water-repellent catalyst is used to reduce influences of steam present in the reaction system). Moreover, it has been reported that hydrophobic silica was developed as an adsorbent for adsorbing organic components in aqueous solutions (see U.S. Pat. No. 4,061,724). In a fuel cell, for example, a fuel cell of the oxygen-hydrogen system, an electrode acts also as a catalyst for promoting electron donating and accepting reactions of oxygen and hydrogen molecules, and such electrode plate is rendered water-repellent by coating with a polytetrafluoroethylene resin or the like to prevent an electrolyte from leaking into a gas chamber.
In Canadian Pat. No. 959,821 and No. 959,628, there are disclosed processes for oxidizing and reducing N.sub.2 S or other substances by introducing an oxidizing gas or a reducing gas into a reaction system in the presence of a hydrophobic catalyst whose carrier or substrate is electrically conductive.
In the known processes, transportation or transfer of electrons which effect oxidizing and reducing reactions is carried out through the electronically conductive carrier or substrate.
The present inventors have found that reducing and oxidizing reactions surprisingly took place when a catalytically active, non-carbonaceous, porous ceramics or plastics is used; the catalyst being repellent or non-philic with respect to a liquid in a reaction system into which a reactive gas is introduced.
The carrier used in the cited Canadian patents are carbon, which are electro-conductive. On the other hand, in the present invention, the electroconductivity of the carrier is an undesirable characteristic because the reaction is limited to the area where three phases (gas-liquid-solid) are formed on the surface of the catalysts. If the carrier is conductive, electrons generated as a result of chemical reaction are absorbed in the carrier whereby the activity of the catalyst is lowered. Accordingly, the present invention employs the catalysts having a non-electroconductive carrier.